1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) compound, a method for synthesizing the same, and an EL material and device having the same, and more particularly, to bisphenylene-spirobifluorene compounds, a method for synthesizing the same, and EL material and device having the same.
2. Description of the Related Art
In general, an electroluminescence (EL) device using low molecular weight materials is formed by sequentially depositing a transparent electrode, an electrically conducting buffer layer, a light-emitting layer, an electron transport layer and a metal electrode on a substrate under high vacuum. If the transparent electrode and the metal electrode are connected to positive (+) and negative (−) terminals of power, holes are supplied to the hole transport layer and the light-emitting layer through the transparent electrode and electrons are supplied to the light-emitting layer through the metal electrode. The holes and electrons supplied to the light-emitting layer are combined therein, thereby achieving luminescence. Since organic EL devices are quick in response, driven at a low operating voltage and self-illuminating, they require no additional back light source, making them small and lightweight. Also, the organic EL devices have excellent luminance and are not dependent upon viewing angle of display.
Types of light-emitting materials forming the organic EL devices include low molecular weight organic materials and organic polymer materials. While the low molecular weight organic material is subjected to vacuum deposition to form a thin film on a substrate, the organic polymer material is molten in an organic solvent to be subjected to spin coating to form a thin film on a substrate. The low molecular weight organic luminescence device is described in detail in U.S. Pat. No. 4,539,507.
Since the color of light emitted is varied according to the band gap of light-emitting molecule, it is quite important to develop red (R), green (G) and blue (B) emission materials having high luminance at low voltages while maintaining high color purity according to the wavelength of light emitted. Development of B emission materials is, inter alia, highly demanded. This is because one of full-color organic EL device implementation methods, that is, RGB emission, can be implemented by forming a blue emission layer, depositing a color change material (CCM) capable of adjusting wavelengths and converting blue emission having the highest energy into green and red emissions.
As materials to emit blue fluorescent light, polyphenylenes having phenylene as a basic unit were reported by J. M. Tour in 1991 (Adv. Mater. 1994, 6, No. 3, pp. 190-198). However, these materials have poor solubility while they are excellent in view of thermal stability and color purity. Also, since it was known that materials having fluorenes, which are pseudo materials of phenylenes, as a basic unit, are promising, a great deal of research into these materials has been made hereto.
Woo, Inbasekaran, Shiang and Roof developed EL polymers using a polymerization technique of polymers, called Suzuki coupling, and disclosed poly(fluorene) based EL materials in WO 97/05184 in 1997. In 1999, poly(fluorenes) with polar side chains were proposed by Pei, Yu and Yang, but the polar side chains promote formation of excimer, which is one of the problems with poly(fluorene), resulting in degradation in color purity (see U.S. Pat. No. 5,900,327 and J. Appl. Phys., Vol. 81, pp 3294-3298). In order to solve the problem of degradation in color purity of blue light-emitting electrochemical cells due to formation of excitons by conventional poly(fluorenes), Chen, Klaerner, Miller and Scott of IBM Corporation developed poly(anthracene-co-fluorene) copolymers obtained by copolymerizing the poly(fluorenes) with anthracenes (see U.S. Pat. No. 5,998,045). As described above, fluorene-based B light-emitting materials with high blue color purity and emission efficiency have been developed. However, planarity of fluorene molecules causes an excimer phenomenon in which the fluorene molecules get easily closer to each other than non-planar molecules, resulting in a change in wavelength of light emitted. The excimer phenomenon makes it difficult to maintain color purity.
In practice, one of B emission materials being currently in widespread use is a compound of distyrylarylene (DSA) based 1,4-bis(2,2-diphenylvinyl)biphenyl (DPVBi) proposed by Hosokawa, Higashi, Nakamura and Kusumoto, in Appl. Phys. Lett. 67 (26), 3853, 1995, and manufactured by Idemitsu Kosan Co., Ltd. Here, the luminance was 6,000 cd/m2 and the emission efficiency was 0.7 to 0.8 Im/W. Also, it was reported that the DSA-based compound improved approximately two times in luminance and emission efficiency, that is, 10,000 cd/m2 and 1.5 Im/W, when it is used with a doping material. The DPVBi used herein has a non-planar molecular structure due to steric inhibition of an arylene substituent.
Accordingly, preparation of spirobifluorene compounds having properties of fluorenes and non-planar molecular structures, and organic EL devices thereof, are being vigorously studied.
Kreuder, Lupo, Salbeck, Schenk and Stehlin disclosed poly(spirobifluorenes) having a spiro framework and copolymers thereof (see U.S. Pat. No. 5,621,131). Also, they disclosed EL materials of new spiro low molecular weight materials or polymers having a spiro framework other than spirobifluorene or having a hetero element substituted for 9,9′-positioned carbon element (see U.S. Pat. Nos. 5,741,921, 5,763,636 and 5,859,211). U.S. Pat. No. 5,840,271 to Lupo et al. discloses methods of synthesizing low molecular weight and polymer compounds of 2,7-diphenylene-9,9′-spirobifluorene and 2,2′-diphenylene-9,9′-spirobifluorene having two identical phenylene groups substituted in 2,7- or 2,2′-positions. Although the structure free of alkyl substituent was effective in maintaining color purity and color stability, the processibility of the low molecular weight or polymer materials was poor. In other words, since such material has poor solubility in organic solvents, its film-forming properties are poor.
In order to overcome the poor film-forming properties, Rietz et al. reported in U.S. Pat. No. 6,132,641 that 2,2′-dialkyl-9,9′-bisspirofluorene having alkyl side chains introduced in 2,2′-positions and its polymer 2,2′-dialkyl-7,7′-poly(9,9′-spirobifluorene) have improved solubility in organic solvents and improved polymer film forming properties in EL device applications.